Floor surface underlayment



FIG. 1 is a top and side perspective view of a floor surface underlayment showing my new design;

FIG. 2 is a top plan view thereof;

FIG. 3 is a bottom plan view thereof;

FIG. 4 is a front elevation view thereof; and,

FIG. 5 is a rear elevation view thereof.

The broken lines represent portions of the floor surface underlayment that form no part of the claimed design. 

CLAIM The ornamental design for a floor surface underlayment, as shown and described. 